Britain's security establishment tried to dissuade Tony Blair from agreeing to the Bloody Sunday inquiry, the former Irish prime minister Bertie Ahern said today. Ahern, who was taoiseach at the time the Saville inquiry was set up in early 1998, also said its creation had helped build nationalist confidence in the Northern Ireland peace process.

The inquiry's long-awaited report into the killing of 14 civil rights marchers by British paratroopers in Derry in 1972 will be published at 3.30pm on Tuesday in Derry and London. The 5,000-page, 10-volume report took 12 years to compile, at a cost of almost £191m.

Ahern said its impact on the peace process had been critical. "It was immensely important because at that time we were trying to build confidence and help the people of Derry, who had been dealing with this for years," Ahern said.

"I had to put a lot of pressure on Tony Blair. All the advice he was getting from securicrats was to not go into a full judicial inquiry. I suppose you could understand why now, with the cost and the time.

"But we had done a submission, the Irish government had done a submission, and we had put a lot at stake in building up nationalist confidence that we would be able to work with the British government and work with Tony Blair. So to have them refuse to give us the inquiry, a full judicial sworn inquiry in front of judges at that time, it would have unsettled the nationalist community and unsettled all the organisations that were in Derry fighting the British for a long time."

Ahern said there would have been a negative reaction throughout nationalist Ireland if Blair had not agreed. "If they had not done that, I think it would not have been able to blossom to what was a very good Anglo-Irish relationship, because it would have been a rejection of what the government here and the dail for that matter, the oireachtas [Irish parliament] had put into, that this was something we really needed to try and restore one of the huge tragedies of the conflict, the Bloody Sunday 1972 issue.

"Because he did it, it helped Anglo-Irish relations; it helped confidence with the government, with the parties and with the northern nationalists."

Ahern added: "It gave confidence in dealing with the British government. It gave confidence that we were going to have one of the big issues that was going to be examined and investigated comprehensively, so it clearly helped. I have no doubt about it that the overwhelming advice to him was not to go down this road; it might have been to do something, but not to go this road. This was a decision he made and it was hugely important."

Martin McGuinness, the former IRA chief of staff who is now Northern Ireland's deputy first minister, today denied claims that he had told Blair an apology from London over Bloody Sunday would be enough. The Sinn Féin MP said the assertion by Jonathan Powell, Blair's chief of staff in Downing Street, that McGuinness told Blair a multimillion-pound inquiry was not necessary was "erroneous."

In his book Great Hatred, Little Room, Powell alleges McGuinness made the observations to Blair during secret talks. But McGuinness said: "The citizens of Derry, to a man and woman, want Saville to make it absolutely clear that the 27 people who were shot on that day – murdered and injured – were completely innocent people and that those people who inflicted those deaths and injuries were the guilty parties." In evidence, McGuinness told the inquiry that on Bloody Sunday he was adjutant of the Derry IRA.